The correct answers are must have been and that.
In an it-cleft sentence, we use the relative pronoun that (not where or which) to connect the emphasized phrase to the rest of the sentence, even when emphasizing a prepositional phrase of place like "through the open skylight."
Additionally, must have been is the correct modal perfect form to express a strong logical deduction about a past event.
Modal verb
If you've ever struggled with the difference between You must do this (strong command) and You should do this (advice) — or It might rain (possible) and It will rain (certain) — you've felt how much modal verbs do in English. They're how the language signals certainty, obligation, possibility, and politeness, and getting them right is what stops your speech from sounding either pushy or wishy-washy.
A modal verb is an auxiliary — can, could, may, might, must, shall, should, will, would — adding meaning around ability, permission, possibility, obligation, or speculation. Always followed by the bare infinitive (can swim, never can to swim), and never inflected for person.
Complex sentence
If your writing is technically correct but reads like a list of short, disconnected statements — I overslept. I missed the bus. I was late. — you've hit the limit of what simple sentences can do. Complex sentences are how you fuse those into one flowing thought (Because I overslept, I missed the bus and was late). It's the single biggest jump in writing maturity.
A complex sentence combines an independent clause with at least one dependent clause: I missed the bus because I overslept. The dependent clause typically signals time, reason, condition, or describes a noun, and is introduced by subordinating conjunctions (because, although, if, when) or relative pronouns.
Relative clause
If you've ever paused over who vs whom vs that vs which — or wondered whether a comma belongs before who — you've hit the relative-clause puzzle. English makes meaning depend on whether the clause is essential information or just extra; one missing comma can flip the meaning of the whole sentence.
A relative clause is a dependent clause modifying a noun, introduced by a relative pronoun (who, whom, whose, which, that) or relative adverb (where, when, why). Restrictive relatives are essential and unmarked; non-restrictive are extra information and set off with commas.
Perfect tense
If you've ever written I am living here for ten years (should be have lived or have been living) — you've hit the perfect tense's main puzzle. English insists that "started in the past, still true now" lives in the present perfect, not the simple present. Get this clear and a whole class of common errors disappears.
The perfect aspect marks completion relative to a point in time, formed with have + past participle: I have eaten (present perfect), She had finished (past perfect), They will have arrived (future perfect). Combinable with progressive aspect (I have been working).
Preposition
If you've ever written I'm interested on you (should be in) or I'm good on football (should be at) — you've hit prepositions' main pitfall. Their choice is mostly idiomatic, not logical, and rarely matches what your native language does. Memorising the right preposition for each common verb and adjective is what stops your speech from sounding subtly off.
A preposition is a small word linking a noun or noun phrase to other parts of the sentence: in, on, at, to, from, with. Marks time, place, manner, or abstract relationships. Choice is largely idiomatic, especially in fixed combinations (depend on, good at, afraid of).
C1 | Advanced
If you've ever sat through a lecture in English, written a complaint letter, or argued a point in a meeting and come out feeling actually understood — not just tolerated — you've felt what C1 looks like. The level matters because it's where most universities, certifications, and skilled-work environments draw their language line.
C1 is the advanced level in the CEFR framework, demanding fluent and flexible language: inversion for emphasis, mixed and advanced conditionals, formal subjunctive, cleft sentences, and complex nominal phrases — all used appropriately across registers.
Difficulty: Hard
If easy and medium questions are clicking but you still feel exposed in real conversation or formal writing, you've outgrown the basics. Hard material is where the gaps you didn't know you had show up: the distractor that "sounds right", the rule that interacts with another rule, the case where context changes the answer. It's where genuine fluency is built.
The Hard difficulty tag marks upper-intermediate to advanced challenges — typically B2 and above. Interacting rules, edge cases, plausible distractors, and contexts that require genuine understanding rather than surface pattern-matching.